Churches, temples, synagogues, mosques, pagodas and monasteries--man-made religious structures of every kind--are so closely associated with religious life that, for many, it may be hard to imagine spiritual expression without some form of sacred architecture.

However, ubiquitous as they are, houses of worship are only recent arrivals in the long history of mankind's religious impulse. For thousands of years, long before the advent of civilization, early man, cradled in nature and believing that the inanimate world was teeming with spiritual entities and forces, worshipped, in fear and wonder, the earth, sky and cosmos without the barriers of walls and roofs.

In his new book, Sky Above, Earth Below, Tucson author and meditation teacher John P. Milton tells us that modern-day spiritual nomads can benefit greatly from the example of their ancient predecessors, writing that taking spiritual practices out of buildings and into nature can break down interior walls, opening us up to illumination about the world and ourselves.

Milton, a former professor of environmental studies and a leading figure in the environmental crusades of the 1960s, who has spent his life exploring the natural world and studying an eclectic mix of spiritual paths, began sensing the mystical possibilities in nature at an unusually young age.

"Even before I could speak," he says, "I remember crawling through blueberry patches in the wild meadows on our hillsides. I quickly discovered Nature was filled with Spirit. ... (I)t seemed absolutely obvious that the church of the Earth was the greatest church of all; that the temple of the forest was the supreme temple."

When he was 7, he convinced his parents to let him spend several days alone in New Hampshire's White Mountains (unfortunately, he doesn't provide details for what must have been an incredible adventure), the first of numerous vision quests he's gone on.

Milton writes that vision quests can sometimes be quite intense, pushing people to the limits of their endurance; a traditional Native American vision quest requires a participant to spend up to five days in an 8-foot diameter circle, engaging in ceremony, meditation and prayer, with no food, clothing and sleep, and little or no water. However, he affirms these solitary sojourns in the wild as powerful tools for spiritual regeneration.

"A vision quest," he tells us, "means surrendering to Nature and Spirit in a sacred way to deepen relationships with the mystery of life, the mystery of you, the mystery of Nature, the mystery of Great Spirit. A vision quest means praying for a vision of the truth of your life."

Milton has certainly had some remarkable experiences. He tells of exchanging spiritual energy with eagles and whales, merging with a brilliant and comforting light after being struck by a thunderbolt, using Tantric teachings to recover from a rattlesnake bite and, once, feeling himself levitate after being startled while meditating.

Milton writes that the chief reason the Earth is so conducive to our spiritual health is because we're an integral part of it.

"Our cells," he says, "our DNA, our tissues and organs, our whole bodies, our energy, our diverse emotions, and our mind, all have coevolved with Nature and Gaia over billions of years. ... (T)his is why many of us feel so at home praying, meditating and doing ceremony in wild Nature."

Milton contends that many of our psychological and social ills stem from having encapsulated ourselves in the artificial chrysalis of civilization, and the greater part of this book details many meditations, visualizations, energy exercises and ceremonies that can help us reconnect to our natural matrix. Happily, for those of us less ascetically inclined, none require the spiritual iron-man approach of a traditional vision quest.

While many spiritual philosophies contain world-denying elements, Milton tells us that the living Earth, "truthful, direct, loving and fundamentally supportive," naturally guides us toward our spiritual depths, and the key to inner development--in nature or out of it--is learning how to relax and attune ourselves to the present moment. As a result, he says, we will automatically experience bliss and become primed for important insights.

This book, open-hearted and life-affirming, is bound to deepen readers' appreciation for nature and its many gifts to us.

"The flower of your true nature," Milton writes, "is no different from the tremendous gift of outer Nature. ... As you discover your integral being, you will be supported by the primal integral being of Gaia."